In this research, the friction stir welding was performed underwater in order to evaluate and improve the mechanical properties of aluminum 2024 alloy. Welding samples were compared with the samples in the air welding. The results showed that samples of the underwater welding in case the rotational speed is 1000 RPM and feed rate is in 80 mm/min, has the highest tensile strength, also they obtain the highest hardness at a rotational speed of 1000 RPM and forward speed of 100 mm/min. Also, an increase of travel speed reduces deformation of the material, heat input and soft area width during the welding process and increases hardness of TMAZ and HAZ. With increasing rotational speed, heat input and soft area width increased. This will, on the one hand, decrease the hardness of TMAZ and HAZ and, on the other hand, increase the stiffness of the disturbance area. Increasing hardness also ultimately improves the tensile strength of the welding joint.